Metal coating

ABSTRACT

A coloured chromate coating is formed by treating a substrate with an aqueous dispersion containing a water soluble dichromate of a metal having a valence greater than one, a pigment and a sufficient concentration of a reducing agent to reduce 70-95% of the hexavalent chromium to the trivalent state and heating the coated substrate to 280-500  DEG F. The dispersion also preferably contains dispersed resin particles in a weight concentration not greater than one part of resin to three parts of the dichromate. The preferred dichromate is that of zinc but magnesium or strontium dichromate may also be used.  The pigment which is present in a weight concentration of 10-75% of the dichromate may be titania, carbon, iron oxide, molybdate orange, ferrite yellow, chromic oxide, chrome yellow, the phthalocyanine pigments or metallic zinc.  The following substrates may be coated:  paper, glass, iron, steel, stainless steel, aluminium, beryllium, cadmium, chromium, cobalt, niobium, copper, lead, manganese magnesium, molybdenum, nickel, silver, tantalum, tin, titanium, vanadium, zinc, zirconium and their alloys and also surfaces which have been oxidized, phosphated, sulphided, chromated or oxalated. The dispersion which is applied to the substrate by immersion, flooding, spraying, roller coating or electrostatic spraying may also contain a wetting agent such as anionic, cationic or non-ionic compounds, e.g. quaternary ammonium compounds of long chain alkyl or aralkylamines, long chain alkyl sulphates and tertiary butyl alcohol.  Acrylic resins such as polymethyl acrylate, polyethyl acrylate and polymethyl methacrylate resins are the preferred type for the aqueous dispersion but other resins such as methyl methacrylate homopolymers, the alkyds, butadiene styrene copolymers, polystyrene, polyvinyl chloride, and P.T.F.E. can be used.  The proportion of resin to reducing agent should be between 1:2 and 6:1 by weight and the resin may contain up to 25% of a plasticizer.  The reducing agents should be such that at temperature below the curing range they are not rapidly oxidized by the hexavalent chromium, e.g. sucrose, citric acid, glycerine, and phosphoric acid.  The metal substrate may be cleaned cathodically and anodically in KOH, rinsed with water, H2SO4 and etched with 2% HNO3 prior to applying the coating dispersion.  Several examples are given of different coating dispersions and methods of application and an electrolytic coating of zinc or tin may be applied to a ferrous base prior to coating with the dispersion.  Lithographic coatings, varnishes, lacquers, enamels and other organic coatings may be applied to the chromate coated substrate.  Specifications 793,242 and 885,675, and U.S.A. Specifications 2,188,864, 2,562,965, 2,634,245, 2,683,698 and 2,768,104 are referred to.ALSO:A coloured chromate coating is formed by treating a substrate with an aqueous dispersion containing a water soluble dichromate of a metal having a valence greater than one, a pigment and a sufficient concentration of a reducing agent to reduce 70-95% of the hexavalent chromium to the trivalent state and heating the coated substrate to 280-500 DEG F.  The dispersion preferably contains dispersed resin particles in a weight concentration not greater than one part of resin to three parts of dichromate.  The preferred dichromate is that of zinc but magnesium or strontium dichromate may also be used.  The pigment which is present in a weight concentration of 10-75% of the dichromate may be titania, carbon, iron oxide, molybdate orange, ferrite yellow, chromic oxide, chrome yellow, the phthalocyanine pigments or metallic zinc.  The following substrates may be coated: paper, glass, iron, steel strip or panels, stainless steel, aluminium, beryllium, cadmium, chromium, cobalt, niobium, copper, lead, manganese, magnesium, molybdenum, nickel, silver, tantalum, tin, titanium, vanadium, zinc, zirconium and their alloys and also surfaces which have been oxidized, phosphated, sulphided, chromated or oxalated.  The dispersion may also contain an anionic, cationic or non-ionic wetting agent and the reducing agents may be such as sucrose, glycerine, citric acid or phosphoric acid.  Acrylic resins such as polymethyl acrylate, polyethyl acrylate and polymethyl methacrylate are the preferred type for the aqueous dispersion but other resins such as methyl methacrylate homopolymers, polystyrene, polyvinyl chloride and P.T.F.E. can be used.  The proportion of resin to reducing agent should be between 1 : 2 and 6 : 1 by weight and the resin may contain up to 25% of a plasticizer.  Lithographic coatings, varnishes, lacquers, enamels and other organic coatings may be applied to the chromate coated substrate.  The compositions may be applied by roller-coating, dipping, flooding, or spraying (including electrostatic spraying).  Specifications 793,242, 885,675, and U.S.A. Specifications 2,188,864, 2,562,965, 2,634,245, 2,683,698 and 2,768,104 are referred to.

Sept 11 1962 l.. K. scHusTER ETAL 3,053,702

METAL coA'rING Filed May 19, 1959= y INV ENT ORS udwg'chuter 22v A1onsoLalJr BY Mmj ATTO EYS Unite tats ate 3,053,702 METAL CATTNG LudwigK. Schuster, Philadelphia, and Alfonso L. Baldi, Jr., Drexel Hill, Pa.,assignors to Pennsalt Chemicals Corporation, Philadelphia, Pa., acorporation of Dela- Ware Filed May 19, 1959, Ser. No. 814,200 16Claims. (Cl. 14S-6.2)

The present invention relates to the coating of metal, particularly forreducing the corrosion of the metal and for giving it a coloredappearance, and represents an improvement in the subject matter of U.S.Patents 2,768,103, 2,768,104, both granted October 23, 1956, and2,773,623 granted December l1, 1956. This application is in part acontinuation of applications Serial No. 587,3 88, led May 25, 1956 (nouU.S. Patent 2,911,332, granted November 3, 1959), Serial No. 666,852,tiled June 20, 1957, Serial No. 708,772, filed January 14, 1958 (nowabandoned but replaced by Serial No. 88,018, tiled February 9, 1961),and Serial No. 738,648, tiled May 29, 1958.

Among the objects of the present invention is the provision of coatingcompositions and methods, as well as coated products, that arerelatively inexpensive yet give very good results in both protectingagainst corrosion and imparting a pleasing appearance.

The above as well as additional objects of the present invention will bemore clearly understood from the following description of several of itsexempliications, reference being made to the accompanying drawingswherein:

FIG. 1 is a partially broken away view of a can coated in accordancewith the present invention; and

FIG. 2 illustrates a baling strap coated in accordance with the presentinvention, the strap being shown in place around a bale, part of thecoating being removed so as to better show it.

Although the application of chromium oxide coatings reduced in situ fromaqueous solutions of chromic acid and compatible reducing agents, asdescribed in the abovelisted U.S. Patents 2,768,103, 2,768,104 and2,773,623, provide very good protection against corrosion, the coatingsso produced are not very decorative. By suitable adjustment of thecoating solution formulations, the coatings can be made brown in colorbut the brown is very frequently mottled by interference fringes thatspoil its appearance, apparently by reason of its fairly transparentnature.

According to the present invention pigments can be incorporated in theabove type of coatings to make them present very attractive appearance,but special provision is required to make the pigments compatible. Alsounless the non-pigment ingredients are adjusted in proportion so as toavoid the prior brown colorations, the effect of all but the darkestpigments is largely lost. In other words, the prior brown colorations,although suiiiciently transparent to be beset by interference fringes,are still capable of seriously masking the effect of pigments, eventhose that impart brown colors.

In order to bring out the above advantageous coloring, the chromic acidis replaced by the water soluble dichromate of a metal having a valencegreater than one. To avoid the brown coloration the proportion ofreducing agent is adjusted so that at least 70% of the hexavalentchromium is reduced to trivalent condition in the final coating.Accordingly, of the 40 to 95% range of reduction disclosed in the abovepatents, only the uppermost portion is used.

Suitable pigments for the purpose of the present invention are the moreinert ones such as titanium dioxide, carbon, iron oxide andv thephthalocyanine pigments such as phthalocyanine blue and phthalocyaninegreen. Also effective is finely divided metallic zinc which, thoughhaving relatively low tinctorial ability, greatly increases thecorrosion protection which the coatings provide. The pigments give theirbest hiding when in very nely divided form, as for example when thepigment particles are about one micron or smaller in size, but theincreased protection by the zinc powder reaches a maximum when thepowder particles are about 50 microns in size. In general, pigment ispreferably dispersed in the coating solution in a proportion of from 10to 75% by weight of the dichromate content. Wetting agents, particularlyof the non-ionic type, are advantageously used to keep the pigmentparticles dispersed during the coating process.

To give a more brilliant appearance and good corrosion resistance thecoatings should also have a minimum coating weight of at least aboutmilligrams per square foot of surface covered. Resins are also bestincluded in the coating, as set forth in the above-identified parentapplications, inasmuch as such additions further improve the corrosionprotection and also make it easier to apply very heavy coatings. Coatingweights as high as 1000 to 2000 milligrams or more per square foot canbe readily applied in this way. Too much resin renders the iinal coatingsensitive to the action of solvents which can cause the color to becomestreaky as Well as dulled. To avoid this the maximum resin contentshould be no higher than 1 part by weight for every 3 parts ofdichromate content by weight in the coating dispersion. Excellentprotection is obtained, however, with resin to dichrornate ratios ashigh as 2 and as low as 0. The best overall results are in the resin todichromate range from 1.5 :1 to 1:3. Wetting agents also preferably ofthe non-ionic type are helpful in keeping the resin particles `dispersedin the coating mixture. The above ratios are calculated by comparing theresin weight to the difference between the weight of the metaldichromate and the weight of the cation of the dichromate.

A feature of the present invention is that even without resin thepresence of pigments improves the corrosion resistance contributed -bythe coating. Phthalocyanine tbluc and TiO2 are particularly effective inthis respect.

Acrylic resins such as methyl acrylate, ethyl acrylate and methylmethacrylate resins are a preferred type of resin. They can be providedin hardnesses of Tukon 2 `or more, and where the coated product is to|be subjected to handling, softer resins, particularly in amounts largerthan 1 part by weight for every 2 parts lby weight of dichromate in thecoating dispersion, cause the iinal coating to become more or lessreadily marked by finger prints and the like. Hard pigments like T102also reduce the susceptibility to marking.

The pigment-containing coatings of the present invention are applicableto any material that can withstand the application conditions. Evenpaper can be so coated. However, the invention is of primarysignificance in connection with the coating of metals by reason of thehigh corrosion resistance which the coating contributes. Indeed, thecorrosion resistance is of such magnitude that the coating will evenprotect such metals as chromiumnickel stainless steels (18-8 forinstance) which are relatively slow to corrode. The coating also adheresvery well to corrodible metals although in some case, as with aluminumand stainless steels, it may be helpful somewhat to roughen the surfaceof the metal slightly, or else not to polish the surface too much whenthe metal is manufactured. Roughening can Ibe effected by etching orabrasion, as with sand blasting, or even -by rolling the metal betweenrough surface rollers. In the case of plain carbon steels and iron,roughening is not needed to improve the adhesion of the coating, but thecorrosion protection of such metals can be sharply increased by givingthem a preliminary grain boundary etch, as described in applicants priorapplication Serial No. 592,552, tiled lune 20, 1956, now Patent No.3,031,333. The coatings of the present invention can also be appliedvery elfectively over metals that have been given a prior adherentcoating of oxide or other materials such as phosphate, oxalate, sulfideor other chromate coatings, including those used to improve the adhesionof paint to ferrous metals as well as to lubricate those metals 'forworking operations. Examples of such preliminary treatments are given inthe above-identified U.S. Patent 2,768,104.

Coatings of the present invention when so applied over the pre-coatedmetals give much better protection against corrosion as compared topre-coated metals when they are not given a second coating. It is alsonoted that a water-base paint when applied over such double coatedmetals, also gives much better corrosion resistance, and for thispurpose the coating of the present invention need not have any resin orpigment present and can even be applied in very thin films. Thus, amilligram per square foot coating from an aqueous solution of only zincdichromate and a compatible reducing agent like sucrose, so as to cause70 to 95% reduction of the hexavalent chromium to trivalent form whenthis solution is subjected to the curing step, makes a very desirablecovering over plain carbon steel that has been phosphatized, as shown inExamples 13 and 14 of the above-identified Patent 2,768,104. Thezinc-dichromate-reducing-agent solution can be merely applied in theform of a final rinse after the phosphatizing operation, particularlywhen the material being treated is a formed object such as an automobileframe or chassis. A similar improvement is obtained with such formedobjects that are first given a grain boundary etch, then coated with thezinc-dichromate-rcducing agent mixture, and after curing, given a finalcoat of water-based paint. Similar results are also obtained with suchformed objects when CrO3 is used in place of zinc dichromate in each ofthe above modifications.

Metals suitable for treatment in accordance with the present invention,in addition to the stainless steels, plain carbon steels and aluminummentioned above, include straight chromium steel, beryllium, cadmiumchromium, cobalt, columbium, copper, lead manganese, magnesium,molybdenum, nickel, silver, tantalum, tin, titanium, vanadium, zinc,zirconium, and alloys of any of these metals with each other.

For avoiding color appearance on very bright surfaces such as stainlesssteel and aluminum, polytetrafluoroethylene can be used in a mixturethat leaves a coating weight of 1.5 milligrams per square foot. therecan be used a coating mixture of 6 grams zinc dichromate 2 grams sucrose4 cc. of an aqueous dispersion of polytetrafiuoroethylene having 50%resin solids content by weight 0.05 gram p-octyl phenyl tri-oxyethylcneethanol diluted to 4 liters with water.

A dip in the above formulation followed by heating of the dipped productat 400 F. to cure the coating left by the dip, provides a 3 milligramper square foot protective layer that imparts very good corrosionresistance to stainless steel articles such as automobile windshieldwiper arms. Most of the cured coating is in powdery form and willreadily rub off on handling, without detracting from the corrosionresistance but improving the appearance by rendering the protectivecoating invisible. In the above formulation the resin-to-dichromateratio can be varied over the 1.5:1 to 1:3 range mentioned earlier, andin fact the resin can be entirely omitted although this diminishes thecorrosion protection somewhat. The resin-free coating does not powderoff but can be butfed off as described in patent application Serial No.666,852. Also the zinc dichromate of the above formulation can bereplaced by strontium dichromate, or calcium dichromate or even For suchtreatment Cr03 with not much loss of effectiveness. The curing can takeplace at any temperature between 250 and 450 F.

The following example gives one typical techniquefor providing thepigmented coatings of the present invention:

Example I An aqueous dispersion is prepared with the followingformulation:

64 cc. of an aqueous solution of zinc dichromate containing 0.75 gramper cc. of chromium as CrO3.

32 cc. of an aqueous solution of sucrose containing 1/a gram of sucroseper ce.

60 cc. of an aqueous dispersion of three parts of carbon black to onepart of phthalocyanine blue, the dispersion containing 710 gram ofsolids per cc. and stabilized with 0.1% addition by weight of hydroxyethyl cellulose.

30 cc. of an aqueous polymethyl methacrylate resin dispersion containing13 grams of polymer having an average molecular weight of 10,000 and aTukon hardness of 4, stabilized with 0.1% of the ammonium salt of4-isopropyl naphthalene sulfonic acid.

The above ingredients are diluted to 600 cc. with water and SAE 1010steel strapping is dipped in it following which the strapping is rolledbetween grooved rollers. The resulting wet metal is passed through anair oven where it momentarily reaches a temperature of 350 F. Theresulting article has a coating that weighs 200 milligrams per squarefoot and has a very pleasing black appearance. It also withstands a 98%humidity test at 95 F. for 100 hours without showing a sign of attack.The coating adheres very well, cannot be scraped off with the fingernail, and does not take any impression by pressing a finger against it.

Similar results are obtained over the entire range of coatingformulations as set out above, except that other resins such aspolystyrenes and the butadiene-styrene copolymers described in parentapplication Ser. No. 708,772, iiled January 14, 1958, will take a markcorresponding to a finger print if they have a Tukon hardness of lessthan about 2. The coatings that are free of resin or contain not morethan one part of marking resin for every three parts of dichromate inthe coating dispersion, do not show marking.

The above coatings also produce a very pleasing leather like appearancewhen applied over metal that has been roughened by rolling with roughrollers, and type 302 stainless steel panels roughened in this way andcoated as per Example I, give an unusually attractive appearance.Although as indicated above curing temperatures of 250 to 450 F. can beused with pigment-free coatings, the higher conversion needed forpigment-containing coatings requires a minimum curing temperature ofabout 280 F.

Although Ti02 is a white pigment, it normally colors the coating apastel green, and when combined with phthalocyanine blue it will providea darker green color. When mixed with carbon black it gives a pleasinggrey. Phthalocyanine green also supplies a green color, red iron oxide ared color, and phthalocyanine blue a striking blue color.

The coating solution can also be applied by spraying, and by adjustingthe spray operation, the aqueous medium of the coating dispersion can becaused to evaporate in considerable proportions before the sprayparticles reach the surface being coated. Thus the addition of air jetssurrounding the spray jet can be arranged to evaporate as much as 3A ofthe water in the traveling of the sprayed particles to the surface beingcoated. As a result, the sprayed particles deposit on the surface in amore or less Viscous form which does not run to any appreciable degreeand furthermore builds up quite readily to give coatings weighing asmuch as 5,000 milligrams per square foot. This type of coating is ofspecial significance in the treatment of fabricated articles whererunning of the coating is to be avoided. Regardless of the coatingthickness, the curing operation can be completed in a few seconds.

Where a large proportion of non-marking resin such as the above acrylateresins are desired and the metal after coating is to be worked as by adeformation operation, it is helpful to plasticize the resins as withpolyvinyl stearate or other plasticizing agents such as acrylate resinshaving molecular weights of about 2000. rIhe polyvinyl stearate can beone produced by reacting polyvinyl alcohol with sufficient stearoylchloride to cause about half of the hydroxyl content of the polyvinylalcohol to become esteried, or can be a fully esterified material suchas made in the manner described in U.S. Patents 2,188,864 and 2,562,965.The plasticizer can be added in an amount up to about 25% of the weightof any plasticized resin and the metal so coated is suitable for highspeed stamping operations such as in the making of tops and bottoms forcans used in conventional canning.

FIG. 1 shows such a can l0. The can has a tubular body 12 made in theconventional manner from a single strip `of a full bright finished blackplate l mils thick with the ends joined together as indicated at l.These ends are folded back on each other to provide interlocking flanges16 and 18 between which a yielding gasket layer 1"/ such as rubber isapplied. The flange and gasket combination is then crimped together tomake a suitably sturdy and leak-proof joint.

The openings at the top and bottom of the tubular body are sealed withcovers 20, 22 that have beaded peripheries 24 with interlocking flanges26, 28. Circular strengthening ribs 36 can also be formed in the top andbottom. The interlocking beaded periphery can merely be crimped,preferably with a layer 30 of gasketing to assure hermetical sealing.

As shown by the broken-away edge 32, the metal of the can body as wellas the top and bottom, has a coating 34 applied in accordance with thepresent invention. This coating was provided on the unfabricated sheetby subjecting it to the following operations:

Example II A. Clean the black plate cathodically in an aqueous solutioncontaining 16 grams KOH per liter using a current density of `amperesper square foot of cathode at 14C-160 F. for l0 seconds.

B. Cold water rinse.

C. Clean anodically in an aqueous solution containing 16 grams KOH perliter using Aa current density of 15 amperes per square foot of anode at140l60 F. for l0 seconds.

D. Cold water rinse. Y

E. Flood with a passivity-preventing 1/2% by weight aqueous H2804solution for 1/2 second at 80 F.

F. Cold water rinse.

G. Spray with an aqueous solution containing 2% nitric acid -at 80 F.for 8 seconds, using a spray pressure of about 6 pounds per square inch.

H. Cold water rinse.

I. Desmut by brushing in water to remove loose or non-adherent reactionproducts including `any developed in step G.

I. Flood for 2 seconds with a 75 F. aqueous solution of:

840 cc. of a 46% solids dispersion of a thermosetting butadiene-styreneresin similar to that described in Example 1 of U.S. Patent 2,683,698but using styrene in place of the mixed vinyl toluenes, and omitting thesodium bicarbonate;

12 grams p-octyl phenyl tri-ethylene oxyethanol;

112 grams sucrose;

375 grams of a 36% TiO2 dispersion in water;

600 cc. of an aqueous Zinc dichromate solution containing 1% gramdichromate measured as CrOg, per cc;

60() grams Zinc dust; all diluted to 6000 cc.

K. Roll through rubber rolls wetted with the ilooded solution.

L. Cure by passing the resulting filmed metal through a drying -unithaving a set of cer-amic gas burners heated red hot by burning gas, afive second exposure to the incandescent units being used, and the metalreaching a temperature of 4350 to 400 F.

If the metal is in the form of an elongated strip, it may be coiled updirectly after step L. The resulting coating weighs approximately 200milligrams per square foot.

The same coated sheet, which -in the above example is colored palegreen, can be used for making both the top and bottom 20, 22 of the can'as well as the body 12 (FIG. 1) by stamping land crimping operation,although in some cases different thicknesses of metal can be used in thedifferent portions of the can so that separate coated sheets arerequired.

After the can body is secured together in the above manner, or bysoldering, the cover on one end may be applied in any convenient mannersuch as the one generally used -in the industry `and shown in FIG. 1.The can can then be filled and the remaining cover applied with theusual precautions where the contents have to be sterilized or heated orkept in a special `atmosphere such as under evacuation orsuperatmospheric pressures. The corrosion resistance of the thus treatedmetal is outstanding and is better than that of tin-plated cans. Thecans having the coating of the present invention are particularly usefulin storing such materials as detergents, soaps and other alkalinecompositions, and are also suitable for holding chemicals yand otheritems that are neutral or acid in reaction.

The stamping of the can tops, =as shown in FIG. l, involves a fairlyabrupt deformation of the margin 318 to provide a backing for the beadcrimping operation. The presence of the pigment does not significantlydetract from the effectiveness of the coating in withstanding suchdeformation. However, when resins `are used in an amount less than onepart by weight for every one part of dichromate by weight, the resin ispreferably of the butadiene-styrene type or contains about l0 to 25%plasticizer.

Another Very effective treatment is with softer acrylate resins, asfollows:

Example III 'Ihe procedure of Example II is followed except that step Jis carried out by rolling the metal between rough rubber rolls wet withthe following dispersion:

48 grams Cr03 reacted with ZnO to form zinc dichromate;

16 grams sucrose;

60 cc. of 26% by weight aqueous dispersion of phthalo'- cyanine blue;

37 grams of methyl acrylate resin having a Tukon hardness of 1.2. and inthe form of a 46% dispersion in water; all diluted to 500 cc.

In addition, the curing temperature is reduced to about 320 F. A veryattractive and protective deep blue finish results.

Inasmuch as cans generally carry lithographed or paper labels which maycompletely cover the body 12 and only leave the top and bottom exposed,the coloring of the present `invention can be limited to only the topand bottom. The remainder of the can may be made from metal coated inother manners such as with the pigmentfree coatings of patents numbered2,768,103, 2,768,104 and 2,773,623 as well `as application Serial No.738,648. The omission of the pigment means that thinner coatings can beused, particularly if of the resin-free type. The coatings on the insideand outside'surfaces of the can body and/or can ends, -need not be thesame. The outer surfaces can, for example, .be of the pigmented typecontaining zinc either as pigment or as a sublayer (see Example IV),while the inner surfaces can have the pigment-free and zinc-freecoatings. The fact that aoearoa good protection can be obtained evenwith extremely thin coatings such as those weighing about milligrams persquare foot, makes such coatings also valuable for the treatment of suchmaterials as filters or the like, and even for such articles astelevision picture tube masks or screens.

lFor color television picture tubes, masks that provide the three-colorseparation for individual element-s of the screen can be made by etchingor otherwise forming ne apertures in `a sheet of metal such as iron orcopper, and the completed mask can be coated with a dilute aqueoussolution of 2% chromic acid and 273% reducing agent such astriethanolamine, after which the mask can be drained to free it fromexcess solution and then heated to 250 F. to cure the coating.Substitution of zinc dichromate for all or part of the chromic acid isto be avoided in such uses inasmuch as such substitution makes it moredicult to adequately outgas the tube. The chromic acid type coatings,even though they appear to be hydrated, do not complicate the outgassingtreatment which the inside of the tubes must undergo.

An exceptionally effective coating according to the present inventionuses a thin undercoat of electrodeposited zinc. A typical technique ofthis type is:

Example 1V SAE 1020 steel sheet is cleaned in a 2% aqueous NaOH dip at180 F., rinsed with cold tap water and then electrolyzed -as a cathodein the following bath:

60 grams per liter of Zn(CN)2; 23 grams per liter of NaCN; and 53 gramsper liter of NaOH.

The current density is adjusted to 11 amperes per square foot ofcathode, the bath temperature to 45 C., and the electrolyzing continueduntil a 150 milligram per square foot layer of zinc is deposited. Thezinc-covered metal is again rinsed with tap water, and sprayed with thefollowing aqueous formulation:

Percent Zinc dichromate 45 Sucrose 8.1 Thermosetting butadiene-styreneresin solids 27.2 Polyvinyl stearate 9.4 TiOZ 9.5 p-Octyl phenyltri-ethylene oxyethanol 0.8

The sprayed metal is cured at 375 F. to leave a coating weighing 200milligrams per squared foot. The resulting coated product withstandscorrosion better than aluminum-painted steel, even in sharply deformedareas.

Other pigmented coatings of the present invention are formed fromformulations such as Example V To 6000 cc. water are added 4 grams ofp-octyl phenyl tri-ethylene oxyethanol 500 grams SrCr2O7 300 gramsphthalocyanine green, and

140 grams citric acid.

This produces dark green `coatings weighing about 250 milligrams persquare foot and having very good corrosion resistance on steel,aluminum, cooper and brass, when cured at 375 F.

Example VI 1000 cc. water 100 grams MgCrzOq made by reacting 7 parts ofMgCO3 with an excess (l1 parts)l of CrO3 40 grams H3PO3 grams red ironoxide 4 grams dioctyl sodium sulfosuccinate.

This produces a resin-free red coating having very good protectivequalities for steels, when cured by flaming so that the steel reaches amaximum temperature of 325 F.

Flaming can also be very efficiently used to cure the resincontainingcoatings of the present invention, and in fact is suitable for drying orcuring any kind of organic coat such as water-based paints,water-dispersed resin, and even paints having organic thinners. Wherethe thinners are combustible most of the thinner should be evaporatedoff before the paint layer reaches the flaming zone.

The coating dispersions of the present invention can tolerate a smallamount of excess CrO3 as indicated by the formulation of Example VI.Resin can also be present along with the excess CrO3 as for instance byadding to the formulation of Example VI 50 grams of the alkyd resin ofExample I in U.S. Patent 2,634,245 emulsied with A the amount ofpolyethylene glycols there described.

FIG. 2 shows an application of the present invention where the coloringis more significant than the corrosion resistance. A bale 40 of cotton,for example, is here shown as held by baling straps 41, 42 coated inaccordance with the present invention. These straps can be of standardtypes of steels such as ordinary SAE 1010 or SAE 1020 steel, preferablygiven an extra severe rolling operation to increase the springiness andhardness of the steel somewhat. Before or after the steel is slit intothe final width, generally 1/2 to 3A inch, it is conveniently coated inthe manner set out above in Example I using a preetch, giving a productthat is very desirable and inexpensive to manufacture. The grainboundary etch sharply increases the corrosion resistance and is valuablewhere straps are exposed to severe corrosion conditions. Although thatetch can be omitted (along with the preliminary passivity preventiontreatment), the industry does expect to have baling straps colored inthe manner that they have been for the past fifty years or more, and itis this color which the present invention provides in a simple,inexpensive and attractive fashion.

The grain boundary etch as with nitric acid in the manner shown inExample II or in U.S. Patent application Serial No. 592,552, led June20, 1956, is also helpful for use with electrodeposited coatings. Tinplated on black plate that has been pre-etched in this manner to remove300 milligrams of metal per square foot, shows much `better corrosionresistance Whether or not the tin plating is covered with the reducedchromic acid or reduced dichromate coating. Unprotected tin plated fromacid electroplating baths is particularly improved by this pre-etchingwhen in plating thickness of from 1/4 to 1 or more pounds per base box,although alkaline deposited tin plate is also improved. Grain boundaryetches that remove from about 50 to about 400 milligrams per square footare very effective in providing this improvement.

The dichromate coating solution or dispersion can be applied byimmersion, flooding, spraying, roller coating, electrostatic spraying,etc. Roller coating, immersing and ooding are particularly suitable forcontinuous coating of strip metal. The thickness of the coating dependsupon the concentration of the mixture, and if applied by roller coating,the roll pressure and roll surface characteristics also affect thethickness of the final coating.

A further feature of the present invention is that thepigment-containing coating dispersions generally do not need anysupplemental wetting agent addition to help wet metals being treated. Itappears that the pigmentcarrying dispersions with sufficient stabilityto hold the pigment suitably dispersed have very good wettingcharacteristics regardless of the metal being coated. This is even moremarked where the dispersions have resin as well as pigment stablydispersed. The use of dispersions is also associated with a drop in thecriticality of the pre-cleaning step.

The dispersing or wetting agents used to stabilize the dispersions canbe of any kind. In addition to those named above, long-chained alkylsulfates are suitable and tertiary butyl alcohol will be effectivealthough it does not cause any appreciable chemical reduction and isgenerally required to be in higher concentration, e.g. 0.5%,

to provide really effective wetting. In general, however, anionic,cationic or non-ionic wetting agents including quaternary ammoniumcompounds of long-chain alkyl or aralkylamines, can be used.

The coatings of the present invention can also be ap'- plied to glassfor the purpose of reducing its attack by etching agents such ashydrofluoric acid. This feature is a great help in the manufacture oftelevision picture tubes with glass envelopes, where the tubes haveexternal metallic supports that are anchored to the glass envelope bymeans of a fritted glass block or disc fused to the envelope andproviding an internally enlarged socket within which a headed-over metalarm is held. By applying the coating of the present invention to theglass frit and if desired to the adjacent portions of themetal arm, thetube envelope can be subjected to the ordinary processing involved inthe conventional manufacture without having the frit attacked to thepoint where its `anchorage to the glass envelope is dangerouslyweakened. Both resin-free and resin-containing coatings can be used forthis purpose, and in addition, the dichromate can be replaced by chromicacid either completely `or partially. Pigments need not be used in suchprotective coatings although they are helpful.

Another `feature of the coatings of the present invention is that theycan be used to reduce the corrosion of plain car-bon steel razor bladesof the type used in shaving, for example. In this connection coatingswhich may or may not have resin are preferably applied in relativelythin layers directly over the cutting edge of the razor. A coatingweight of only about 30 milligrams per square foot is adequate yfor thispurpose, and such coatings are helpful in reducing somewhat the initialkeeness of the edge. As a result, the tendency for a shaver to cut hisface with a brand new blade is greatly diminished. However, thecorrosion protection preserves the cutting edge between shaves so thatit can Ibe used for at least as many shaves as unprotected edges.

As indicated above, the coatings of the present invention are alsoapplicable ras top coatings to sharply increase the protection of metalgiven a preliminary coat with other types of protective material. Suchpreliminary coatings can be of the phosphatizing type or of the oxidetype, or of the standard coloring preparations such as those used forcoloring zinc black.

In addition, because of the purely inorganic nature of the resin-freecoatings of the present invention, the coatings can be applied toprotect articles that have a temperature too high for other types ofprotection. Instances of such use are metal castings which can begin tooxidize after they are removed from the casting mold and during the timethey cool down to room temperature. Aluminum or Zinc castings canaccordingly be sprayed with the coating dispersions of the presentinvention when their temperatures are as high as 500 F. or even higher.These high temperatures cause the coating to be promptly cured, and whenthe temperature is very high can begin to decompose the coating.However, even as so decomposed, oxidation of the metal is suflicientlyinhibited during the cooling process as well `as for days thereafter. Asmall amount of resin can even be present in such coatings and nopigment need be used. For such protectionof pigs used to replenishmolten metal baths, it is also preferred to apply `only a very thincoating using simple solutions of chromic acid and polyalcohol reducingagent such as sugar. Less foreign material is introduced into the bathin this way, although not much is added by replacing the CrOs with oneof the dichromates.

Throughout all the above ranges of formulation, the proportion ofingredients is always adjusted so that at least 40% and not more than95% of the hexavalent chromium becomes converted to trivalent chromiumwhen the curing is completed. However, as pointed out above, thepigment-containing mixtures should stay within the range of 70% to 95conversion. A pigment like l0 TiO2 is particularly desirable in that itincreases the ductility o-f the coating .so that it better protectsmetal that is subsequently drawn or pressed. Accordingly, when a greenor blue color is desired, the colored pigment is advisedl-y used incombination with a liberal amount `of TiO2. In Example II, for instance,Ithe `substitu-tion of parts of phthalocyanine green lfor an equalamount of the TiOZ gives a good green color on a product that is morefully protected in .areas that are to be deformed. The presence of theTiOz also gives the coating better hiding qualities and .as pointed outabove, makes the coating harder than TiOTfree pigments do.

`Obviously many modifications and variations of the present inventionare possible in the light of the above teachings. It is, therefore, -tobe understood that within the scope of the appended claims the inventionmay be practiced otherwise than as speciiically described.

What is claimed:

1. A method for simultaneously protecting and coloring a metal, saidmethod being characterized by the steps of applying to a surface of themetal `an aqueous dispersion of:

(a) a water-soluble dichromate of a metal having a valence greater thanone;

(b) a compatible reducing agent 'for the dichromate;

(c) a resin; and

(d) a pigment,

the reducing agent being one that causes the chromium of the dichromateto be reduced to trivalent condition when the dispersion is heated to atemperature of at least about 280 F., the concentration of the reducingagent being such that from about 70 to 95 of the chromium is so reduced,and the pigment being in a concentration of about 10 to about 75% byweight of the dichromate, and heating the metal so Icoated to atemperature between about 280 di'. and 450 F. to dry the applieddispersion and cause the above reduction, the quantity of dispersionapplied to the metal being adjusted to give a coating weight after theheating of at least 100 milligrams per square foot.

2. The method of claim 1 in which the pi-gment is selected from theclass consisting of phthalocyanine pigments, TiOZ, car-bon, and ironoxide.

3. The method of claim 1 in which the dispersed resin is in aconcentration by weight not greater than twice the weight of thedichromate.

4. The method of claim 3 in which the dispersion is applied by spraying.

5. The method of claim 4 in which the `spraying is carried out wi-th astream of compressed gas directed to evaporate olf at least aboutthreefourths of the dispersant before the spray reaches the metalsurfaces, and the coating weight after the heating is at least 1000milligrams per square foot.

6. The method of claim 1 in which the metal is plain carbon `steel andthe dispersion contains metallic Zinc particles less than 50 microns insize.

7. The method of claim 1 in which the dichromate is zinc dichromate.

8. The product produced by the process of claim 1.

9. The method of claim 1 in which the metal is plain carbon steel andthe surface to lwhich the dispersion is applied has a grain boundaryetch.

10. The method of claim 1 in which the resin is an acrylic resin.

11. The method of claim 7 in which the drying and reduction of thecoating is completed in not more than five seconds.

v12. The method of claim 7 in which the resin is an acrylic resin andthe drying and reduction is effected by momentarily bringing the coatedmetal to the heating temperature.

13. In the method of protecting metals against corrosion by applying anaqueous solution of a member selected from the class consisting ofchromic acid and dichromates of metals having a valence greater than 1,said solution also containing a reducing agent compatible With saidmember, the proportion of the reducing agent being such that the appliedsolution upon curing at a temperature of 250 F. is converted to acoating in which from 40 t0 95% of the chromium is in trivalent form,the improvement by which said solution is applied to a casting of ametal of the class consisting of aluminum and zinc while said casting isat a temperature of approximately 500 F. to cause the coating to bepromptly cured and partially decomposed.

14. In the method of protecting a metal against corrosion by applying tothe surface of the metal an aqueous solution of chromic acid and areducing agent compatible with the chromic acid at room temperature butwhich on drying and curing of the applied solution reduces 40 to 95 ofthe chromium of the chromic acid to trivalent condition, the improvementaccording to which the cured layer is made to present a pleasing colorby substituting for the chromic acid the dichromate of a metal having avalence greater than one, dispersing in the aqueous solution a pigmentin an amount from about 10 to about 75% by weight of the dichromate, andcarrying out the curing at a temperature of at least 280 F. so that atleast 70% of the chromium becomes reduced. 15. The product produced `bythe process of claim 14. 16. The combination of claim 14 in which thedrying and curing of the coating solution is completed in not more thanvc seconds.

References Cited in the le of this patent UNITED STATES PATENTS1,965,269 Tostervd July 3, 1934 2,163,768 Tanner June 27, 1939 2,206,064Thompson et al. July 2, 1940 2,210,850 Curtin Aug. 6, 1940 2,347,572Martin et al. Apr. 25, 1944 2,442,195 Clenny May 25, 1948 2,562,117Osdal July 24, 1951 2,768,103 Schuster et al Oct. 23, 1956 2,768,104Schuster et al. Oct. 23, 1956 2,773,623 Schuster et al. Dec. 11, 19562,853,406 Schuster et al. Sept. 23, 1958 2,902,390 Bell Sept. 1, 19592,911,332 Schuster et al. Nov. 3, 1959 UNITED STATESl PATENT OFFICECERTIFICATE OF CORRECTION Patent Nos 3O53q702 September Il i962 LudwigK3 Schuster et aL,

It is hereby certified that error appears in the above numbered patentrequiring correction and that the said Letters Patent should read ascorrected below.

In the grantv line 4, and in the heading to the printed specificationglines 5 and 6, for "a corporation of Delaware"q each occurrence read acorporation of Pennsylvania Signed and sealed this 16th day of Aprill963 (SEAL) Attest:

4ERNEST w. SWIDER DAVID L- LADD Attesting Officer Commissioner ofPatents

1. A METHOD FOR SIMULTANEOUSLY PROTECTING AND COLORING A METAL, SAIDMETHOD BEING CHARACTERIZED BY THE STEPS OF APPLYING TO A SURFACE OF THEMETAL AN AQUEOUS DISPERSION OF: (A) A WATER-SOLUBLE DICHROMATE OF AMETAL HAVING A VALENCE GREATER THAN ONE; (B) A COMPATIBLE REDUCING AGENTFOR THE DICHROMATE; (C) A RESIN; AND (D) A PIGMENT, THE REDUCING AGENTBEING ONE THAT CAUSES THE CHROMIUM OF THE DICHROMATE TO BE REDUCES TOTRIVALENT CONDITION WHEN THE DISPERSION IS HEATED TO A TEMPERATURE OF ATLEAST ABOUT 280* F, THE CONCENTRATION OF THE REDUCING AGENT BEING SUCHTHAT FORM ABOUT 70 TO 95% OF THE CHROMIUM IS SO REDUCED, AND THE PIGMENTBEING IN A CONCENTRATION OF ABOUT 10 TO ABOUT 75% BY WEIGHT OF THEDICHROMATE, AND HEATING THE METAL SO COATED TO A TEMPERATURE BETWEENABOUT 280* F. AND 450* F, TO DRY THE APPLIED DISPERSION AND CAUSE THEABOVE REDUCTION, THE QUANTITY OF DISPERSION APPLIED TO THE METAL BEINGADJUSTED TO GIVE A COATING WEIGHT AFTER THE HEATING OF AT LEAST 100MILLIGRAM PER SQUARE FOOT.